Fabrication of electrical interconnects and contacts, also called metallization, is performed by a variety of known methods such as electroless and/or electro plating, sputtering, ion beam deposition, evaporation, screen printing, stencil printing, inkjet printing, aerosol jet printing, and/or chemical-vapor deposition (CVD). To efficiently extract electrical power generated from exposure to sunlight, solar cells require low-loss electrical interconnects and contacts to the cell electrical terminals (emitter and base regions). These methods, though sufficient for the formation of certain metallization designs, may be costly, particularly for applications in high-efficiency solar cell architectures, because of one or a combination of the following aspects:                Equipment capital cost and complexity—metallization fabrication equipment and methods are often costly and the processing may be rather complex, particularly for vacuum deposition methods such as physical vapor deposition (PVD) processes (including plasma sputtering, ion beam deposition, and evaporation methods), which may require relative low base pressures and high vacuum environment;        Material utilization—known methods may suffer from relatively low utilization of the primary raw metallization materials for metallization resulting in additional metallization materials required and a higher cost of raw materials used, for example vacuum deposition techniques such as plasma sputtering with finite target material utilization; and        Cost of consumables—these materials include the fabrication materials or consumables other than the primary raw materials, such as chemicals, filters, regeneration of resins used in plating methods, and the cost of manufacturing metal targets used in plasma sputtering technique.        
Atmospheric-pressure thermal and/or arc spraying of metals is a cost-effective method to coat surfaces with specific metals that has been used in a variety of industries, primarily for structural and corrosion-resistance coating applications. While the use of metal spraying technology to form electrical interconnects and metallization contacts on traditional front-contact solar cells has been proposed as early as 1982, complications arising from various factors such as relatively high electrical resistivity of the deposited metal (i.e., much higher resistivity compared to bulk resistivity of the material being deposited) due to porosity and oxidation of the deposited material layer (since deposition occurs in oxidizing air ambient), as well as the adverse effects of impinging high-temperature molten metal directly onto the exposed semiconductor solar cells, such as the direct reaction of hot metal droplets with the semiconductor layer (leading to the killer shunts in the solar cells), as well as the need for fine metallic patterns has prevented the development and commercialization of automated high-productivity spray processing equipment in solar cell manufacturing applications. Most common silicon based solar cells are structured using a single layer metal for each of the contacts and using sprayed metals, especially in an oxidizing ambient, present significant challenges for forming ohmic contacts between the sprayed metal and the silicon contact regions in the cell. Further, fine line metallization requirements, often essential in solar cells, are difficult to meet when spraying a single layer of metal using known methods.
Further, the production throughputs of known thermal or arc spray tools are often too low to enable low-cost metallization for solar photovoltaic applications. Moreover, the electrical resistivities of the conductive materials deposited by known thermal or arc spray tools and processes may be significantly higher than the bulk resistivity values of the materials being deposited (typically by at least an order of magnitude or more) due to the uncontrolled air ambient (resulting in oxidation of the hot metal droplets in air as well as incorporation of other impurities in the droplets), micro-voids, and the porosity of the deposited layers. As a result, for a specified maximum allowable metallization sheet resistance, the required thickness of the metallization material layer is significantly larger than the metal thickness deposited by other methods such as the vacuum PVD techniques. Therefore, a very thick layer (e.g., typically at least from 10's to 100's of microns) of metal would be required to meet the conductivity requirement which may result in relatively high stress, wafer bow, yield and reliability issues, as well as relatively high metallization cost. These factors often make known thermal and arc spray tools and methods unattractive and uneconomical for metallization applications in mass production of microelectronics, solar photovoltaics, and other semiconductor devices.
Additionally, known thermal and arc spray tools and processes may result in very rough surfaces and poor uniformity control for the deposited films and often do not provide any capability for the deposition of in-situ-patterned layers. Thus, patterning of the deposited metal layer often has to be performed by printing of a patterned resist layer such as by screen printing or lithography and subsequent wet chemical etching of a relatively thick rough metal layer—thus resulting in additional manufacturing costs and wastes.
Solar cell developments utilizing the surfaces of a suitable substrate, herewith called a backplane, attached to a thin-film (or ultrathin) crystalline solar cell (for example having a crystalline silicon layer thickness of range of about 1 micron up to less than 100 microns) have necessitated the need for and may highly benefit from an automated high-productivity processing equipment using thermal and/or arc spray metallization applications.